A. Field of the Invention
This invention relates to apparatus for collecting and focusing onto a detector optical radiation in the ultraviolet (UV) through infrared (IR) portion of the electromagnetic spectrum, and to apparatus for receiving and transmitting radio frequency (RF) energy. More particularly, the invention relates to a dual mode apparatus in which are combined the functions of an optical detection system, and a radio energy frequency detection system.
B. Description of Background Art
Radar and laser transceivers are widely used in both military and civilian applications to determine the position and velocity of distant objects, or "targets." Radar or laser acquisition and tracking systems are "active" systems, in the sense that they transmit energy from a radiating element or antenna, which is then reflected off of a target back towards the element or antenna. Some radio frequency acquisition and tracking systems are also passive, utilizing either intentionally transmitted energy, or the black-body radiation emitted by all objects at a temperature above absolute zero.
Passive optical radiation receivers are also widely used to form images of distant objects, utilizing the radiation reflected from or emitted by objects. Such systems typically operate in a wavelength range extending from about 0.35 micron (micrometers), in the ultraviolet portion of the spectrum, through the visible spectrum, and out to about 14 microns, in the infrared portion of the spectrum.
In some applications, it is desirable to be able to acquire and/or track distant objects, or targets, alternatively in the radio-frequency or UV-IR portion of the electromagnetic spectrum. Passive UV or IR tracking systems do not require transmission power, and may operate at longer ranges than radar trackers. However, when a target is obscured by clouds, fog, smoke dust or the like, such tracking systems may be inoperative, thus necessitating the use of an active or passive radio frequency tracker.
Both RF and UV-IR trackers generally must work with very low energy signals emitted by or reflected from a distant target. Therefore, receiving antennas or optical systems used to collect such signals should have the largest feasible collection area, or aperture. Increasing the aperture size is also very desirable because it results in a narrower transmitted beam, and reduces the susceptibility of the receiver to interference from off-axis, non-target emitters such as jammers, or reflective objects other than the target. Accordingly, dual mode RF/IR or UV tracking systems require a considerable amount of space for the required collection optics. That space requirement can be particularly troublesome when the dual mode system is to be used in an aircraft or missile. To alleviate the space requirement problem, devices in which energy collectors for radio frequency and infrared radiation are at least partially combined are disclosed in the following U.S. patents:
Cushner, U.S. Pat. No. 3,165,749, Jan. 12, 1965, Microwave Transmissive Optical Radiation Reflectors, which discloses a Cassegrain infrared sensor system employing a microwave transceiver antenna horn at the position of the secondary mirror. The horn opening is covered with a microwave transmissive, optically reflecting "dichroic" mirror fabricated by forming a hexagonal or linear array of optically reflecting metallic mirror segments on a glass, quartz, or other microwave-transmissive substrate, microwave energy being transmittable through gaps between segments.
Winderman, et al., U.S. Pat. No. 4,282,527, Aug. 4, 1981, Multi-Spectral Detection System with Common Collecting Means, which discloses a multi-spectral detection system for detecting radiation within frequency bands that are in diverse portions of the electromagnetic spectrum. The preferred embodiment is a Cassegrain system including a primary reflector for reflecting radiant energy in the radio frequency and infrared portions of the electro-magnetic spectrum; a subreflector positioned along the axis of the primary reflector for re-reflecting the radiant energy reflected from the primary reflector toward the primary reflector and focusing the re-reflected radiant energy in a common focal plane about the system axis; a feed horn coaxially positioned at the common focal plane and having an entrance in the common focal plane about the system axis for transferring the collected radio frequency radiant energy from the focal plane to a detector of radio frequency radiant energy; and a bundle of optical fibers coaxially positioned at the common focal plane and having openings in the common focal plane about the system axis for transferring collected infrared radiant energy from the focal plane to a detector of infrared radiant energy. A baffle is coaxially positioned for blocking light other than the re-reflected infrared radiation from being received and transferred by the optical fibers.
Brumbaugh, et al., U.S. Pat. No. 4,477,814, Oct. 16, 1984, Dual Mode Radio Frequency-Infrared Frequency System, which discloses a dual mode RF-IR sensor system in which the primary mirror of a Cassegrain system has a central polished and aluminized region to reflect IR energy, and four slots to transmit RF energy. The secondary mirror is made of any suitable rigid RF-transmissive plastic, the reflective surface of the secondary mirror being covered with "any suitable IR-reflective/RF transmissive dielectric coating." A parabolic body forms both the primary mirror for a Cassegrain infrared sensor, and a radiating/receiving body for microwave energy, utilizing a common surface. Only a central portion of the parabolic body is polished, aluminized and used as an IR reflecting surface, and has integral slots forming part of a strip line antenna.
Droessler, U.S. Pat. No. 4,866,454, Sep. 12, 1989, Multi-Spectral Imaging System, which discloses an infrared sensor and millimeter wave (MMW) transceiver apparatus having a common collecting aperture and coaxial elements. The IR sensor includes a Cassegrain optical system in which incoming rays are reflected forward off the outer annular surface of a parabolic primary mirror, backward off of an IR-reflective, RF-transmissive thin film on the convex surface of a secondary reflector, and through an IR-transmissive, RF-reflective coating on the front concave surface of a circular core section fitted into a central aperture in the reflector, to a rear IR detector focal plane.
In the dual mode system, received MMW rays impinging on the outer annular surface of the primary reflector are reflected forward to impinge on the thin film coating on the secondary reflector and therethrough to a feedhorn. Received MMW rays closer to the optical axis of the apparatus pass through the secondary reflector, through the film and are reflected forward off the coating on the central core of the primary reflector, to impinge on the front coating of the secondary reflector and travel through the coating to the feed horn.
Simadao et al, in U.S. Pat. No. 3,763,493, Oct. 2, 1973, Antenna Device Applicable for Two Different Freauency Bands, which discloses an antenna apparatus adapted to simultaneous operation at two different radio frequencies.
Some of the dual mode seekers listed above may be useful for certain applications. However, all of the prior art dual mode seeker systems known to the present inventor have an inherent limitation, namely the fact that none of the systems utilizes the full aperture of a collecting system for either or both radio frequency and ultraviolet or infrared energy transmission and reception. The present invention was conceived of to overcome the above-mentioned limitation, and to provide other advantages over prior art dual mode seekers.